The Relation of Treatment and Ecological Effects in Brackish Water Regions

The ecosystems in brackish water regions are already exposed to stress, especially those living in estuaries with their fluctuating environment. The question raised here is whether these ecosystems are generally more susceptible to additional stress such as pollution, or if their ability to adapt to environmental changes leads to increased tolerance to pollution. The Baltic, as being the largest brackish water area in the world, is discussed with respect to its eutrophication in relation to pollution discharge. A comparative analysis of the effects of pulp and paper mill effluents in the brackish water environment is made between a polluted area in the Bothnian Sea and an estuarine area under recovery on the Swedish west coast. The question of stress tolerance of ecosystems is extremely important when it comes to predicting environmental and ecological effects. It is suggested here that, in general, already stressed ecosystems are more tolerant to pollution than those existing in stable environments, and thus, the brackish water environment is less susceptible than similar marine environments. Discharge of polluted water brings about disorders in aquatic ecosystems. Severe pollution disrupts ecosystems and reduces them to poor levels of organisation. Waste water is often transported to sea by river water. Estuaries are transition zones outside river mouths, where fresh water and marine water meet. However, the water here is simply diluted sea water, which is the same as brackish water. According to the Venice Classification System, brackish water has a salinity of between 05 and 30 ppt. Brackish water regions cause stress to the ecosystem because of their fluctuating environmental conditions. This stress reduces the species diversity. If we increase this stress by adding polluting wastes to the brackish water regions—which is commonly done today—this leads, according to most authorities on pollution problems, to more severe consequences for the ecosystem than if we pollute similar marine regions. This is a question of stress tolerance of ecosystems which is extremely important when it comes to predicting the environmental and ecological effects. The largest brackish water sea in the world is the Baltic Sea, covering 366,000 km2. It is also one of the worlds largest polluted areas. The surface salinity is 8 ppt in the southern parts, decreasing gradually northwards to 2—3 ppt in the Bothnian Bay. The flow of nutritious waste water from communities and industrial plants has tended to increase the biological production, particularly in coastal waters. The major part of organic waste emitted to the Baltic from Sweden originates from the forest industry.1 However, during the last couple of years, a significant decrease in the discharge of organic waste from the pulp and paper industry has occurred.2 If these coastal discharges are added to the wastes transported by rivers, we arrive at a total Swedish contribution to the annual load on the Baltic in the order of 60,000 tons of nitrogen, 7000 tons of phosphorus and 1,500,000 tons of organic matter. The oxygen situation in the deep basins of the Baltic proper has deteriorated during the twentieth century due to, among other things, a rise in the salinity and temperature of the bottom water.3 However, the occurrence of hydrogen sulphide has not yet the permanent character of that in, e.g. the Black Sea; in the P.A.C., Vol. 45, No. 3/4—F 199 Baltic it varies with the exchange of water with the North Sea. The alternatic oxic and anoxic conditions in the bottom water is proposed to be the driving force behind an increasing eutrophicatin of the Baltic. During stagnation periods, decaying organic matter is accumulated in the deep water. When the conditions of the stagnant deep waters change from oxidising to reducing as a result of this decay, nutrients—especially phosphorus— are dissolved from the sediments. Through upwelling, these released nutrients may then be transported up to the surface, where photosynthesis can take place. This process is also reflected in the increasing concentrations of dissolved phosphorus in the surface water of the Central Baltic Basin observed from 1950 to l968. Periods of stagnation under anoxic conditions in the deep basins is most probably a natural phenomenon, and it has recently been shown to be correlated with paleo-climatic variations. The increased pollution during the last decades has perhaps accelerated this process. Local damages in the Baltic ecosystem may be reversible, but a general disturbance of the total ecosystem in such a closed sea area as the Baltic may be irreversible and must therefore be prevented. I will now give two examples of the effects of industrial waste water on brackish water ecosystems. One example is from the Bothnian Sea in the Baltic, the other from an estuary on the Swedish west coast. The waste water in both cases was discharged from pulp and paper industries, and the effects were studied by examining the benthic fauna. This fauna contains, for instance, clams, polychaetes and crustaceans living in tubes or burrows in the top layer of the sediments or on the surface of the sediments. They can be very abundant, in numbers of several thousands per m2. The benthic animals form communities with restricted organisation. They live in the same spot for years and, thus, changes in the community structure are indications of environmental changes in that particular area. Analysis of the benthos can therefore be used to assess the degree of pollution in an area. The first example is from the Bothnian Sea (Fig. 1), (see Rosenberg et al.4). 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